The present invention relates to a process for determining the overrun force which is produced by a trailer on a tractor during the braking operation. According to the invention, the term "trailer" relates to a conventional trailer, which is coupled by a drawbar with the trailer coupling of a conventional truck as well as to a semitrailer which is connected with a fifth wheel of a semitrailer tractor.
In the case of a truck or a semitrailer tractor, the brake pressure required for the braking is supplied to the trailer (or the semitrailer) by an air pressure line or the like. In an ideal case, this brake pressure is to be adjusted or regulated such that the trailer decelerates at exactly the same as its tractor. A brake action of the trailer and the tractor which is coordinated in such a manner has the significant advantage that the braking operation can be controlled very precisely and neither the trailer nor the tractor will jack-knife. At the most, it is permissible to select the brake deceleration of the trailer slightly higher than that of the tractor because in such a case, the braking operation can still be mastered, on the one hand, and the more maintenance-intensive brakes of the tractor are correspondingly conserved, on the other hand.
Since, in the course of time, the braking performance of the brakes of the trailer changes to a different extent than that of the tractor, the relationship of the braking performance of the trailer and the tractor can be ensured only if relatively short service intervals are maintained. Furthermore, it should be taken into account that the weight of the trailer changes as a function of the respective load so that a precise adjustment will only be possible when a so-called automatic braking device ("ALB") is present, which is true only in the case of modern trailers. In order to avoid these disadvantages, it has recently been endeavored to keep the braking force or braking performance of the trailer constant at a respective optimal value by regulating the brake pressure supplied to the trailer as a function of the braking performance of the tractor.
However, such regulating of the braking performance of the trailer is possible only when the force acting on the coupling between the semitrailer and tractor is measured during the braking acts. This force effect of the trailer, which is normally called "overrun force" should therefore, in the above-mentioned ideal case, be equal to zero or at the most should have a slightly negative value. When the overrun force is known or measurable, it is therefore relatively simple to change the brake pressure of the trailer until the overrun force finally reaches the desired value.
However, for measuring the overrun force, a force transducer or load cell must be installed at the corresponding coupling. Because of the very high value, which the coupling force may assume in practice, the price for such a force transducer or sensor may be comparatively high and may therefore be in the order of several thousand Deutsche Mark. Despite the advantages associated with regulating the braking performance of the trailer, this has largely not be implemented because of these high costs.
The invention is based on the object of further developing a process wherein that the respective overrun force can be determined at significantly lower cost.
The invention is based on the idea that the respective or momentary value of the overrun force may possibly also be derived from the measuring signals which already exist in the case of modern tractors, without measuring the overrun force directly by a force transducer. Thus, a force transducer is not required and the costs for determining the overrun force are limited to the use of a correspondingly constructed or modified control unit. Since, particularly when a central computer is available--which is always the case in modern tractors--, the overrun force can be computed only by a suitable change of the central control program, it may even be possible to reduce the costs for this purpose to a negligible value.
For achieving the above-mentioned object, the measurable value which already exist on the tractor be used in such a manner that (a) the equilibrium of the forces is determined which act on the tractor in a vertical or y-direction, (b) the equilibrium of the forces is determined which act on the tractor in the horizontal or x-direction, and (c) the overrun force is determined from the determined vertical and horizontal equilibrium of forces. It was found that the measurable values which exist in the case of modern tractors make it possible to relatively precisely calculate the respective value for the overrun force by means of these equilibria of forces. In detail, the following processes are preferred by the invention:
If the tractor is a conventional truck with a trailer coupling, that is, it is therefore no semitrailer tractor, so that consequently the overrun force (in the following called F.sub.k) of the trailer acts exclusively in the horizontal or x-direction, the overrun force F.sub.k is determined by the following equation: ##EQU1##
In this formula, F.sub.1 is the front axle load of the truck; F.sub.2 is its rear axle load; F.sub.a is the braking force; a is the braking acceleration; and g is gravitational acceleration. Because of the air suspension, which is now customary in modern trucks, the values F.sub.1 and F.sub.2 of the front axle load and the rear axle load can easily be detected and measured from the air pressure of the suspension at the respective axle. These quantities of the equation are therefore known. Likewise, from the position of the brake pedal or from the momentary braking pressure, the value F.sub.a of the braking force can be determined. Finally, the actual (braking) acceleration is also known on the basis of the output signals of the sensors of the antiblock system (ABS). Since the gravitational acceleration g is a constant, the value of the overrun force can therefore be determined from the known equation without the use of a corresponding force transducer or force sensor.
Although, as explained above, the value F.sub.a of the braking force can be determined, for example, from the brake pressure, in practice, there is the problem that this measurable quantity permits only an indirect determination of the actually applied braking performance. The reason is that, depending on the wear and the temperature of the brake, the actual braking performance will deviate from a specific desired value so that the value F.sub.a is not completely certain. In order to keep the influence of the uncertainty of the measurable value F.sub.a on the result of the calculation as small as possible, it is suggested according to the further development of the invention to also determine the equilibrium of moments relative to the center of gravity (in the following called S) of the truck. In this case, it is possible to determine the overrun force F.sub.k by the following equation: ##EQU2## wherein x.sub.t, x.sub.2 and y.sub.12 are the respective relative position of the axles of the truck to its center of gravity S, and y.sub.ak is the vertical distance of the coupling to the wheel axle. The value y.sub.ak of the vertical distance of the coupling to the wheel axis is a function of the geometry of the vehicle and is therefore a constant which must only be put in once (that is, an input value).
The values x.sub.1 and x.sub.2 can be determined according to a special aspect of the invention in a simple manner. The equilibrium of moments is determined at the point in time at which the truck rolls in the unaccelerated condition because in this case, a simple calculation is possible from the distance between axles or wheel base, as indicated in detail in the description of the figures. The rolling condition of the truck can be derived in a simple manner, for example, from the output signals of an electronic diesel injection device ("Electronic Diesel Control--EDC") and therefore represents a measurable value. It may also be possible to determine the values of x.sub.1 and x.sub.2 in the same manner when the truck is stopped or parked. However, in this case, it must be taken into account that this measurement will furnish correct values only when a freedom from forces exists in this stopped condition; that is, the truck must, for example, be completely uncoupled.
In contrast, the relative vertical position y.sub.12 of the axles of the truck can be determined by the following formula: ##EQU3##
Since the slightly uncertain value F.sub.a enters comparatively little into this formula, the calculation of y.sub.12 and therefore specifically that of F.sub.k is relatively precise. Furthermore, according to another essential aspect of the invention, the mentioned formula for y.sub.12 may also be calculated several times and the average value may then be determined.
If the tractor is a semitrailer tractor having a fifth wheel coupling, the overrun force F.sub.k of the semitrailer therefore acts in the horizontal as well as in the vertical direction and therefore comprises two components F.sub.kx and F.sub.ky. Thus, for the regulating of the braking force of the semitrailer, the ratio F.sub.kx /F.sub.ky of the horizontal to the vertical component of the overrun force can be compared to the ratio a/g of the vehicle acceleration a to the gravitational acceleration g. The reason is that, if the determined ratio F.sub.kx /F.sub.ky is approximately almost as large as the ratio a/g, the braking performance of the semitrailer is adjusted to be exactly correct, while a negative or positive result of the comparison characterizes a running-up or a pulling semitrailer.
With respect to details, the F.sub.kx /F.sub.ky ratio can be determined by the following formula: ##EQU4## wherein F.sub.1 is the front axle load of the semitrailer tractor; F.sub.2 is its rear axle load; m.sub.z is its mass; F.sub.a is the braking force; a is the braking acceleration; and g is the gravitational acceleration. These quantities are either constants or are known as measurable quantities, as explained above with reference to a normal truck.
In the plurality of cases, the semitrailer tractors have an air suspension only on the rear axle, while only a leaf-spring suspension or spiral-spring suspension is provided on the front axle. Accordingly, no measurable value of the quantity of F.sub.1 is available at the front axle. In this case, it is possible to determine the desired ratio F.sub.kx /F.sub.ky of the horizontal to the vertical component of the overrun force while also taking into account the equilibrium of moments, in which case a knowledge of the force F.sub.1 is not required. In this case, the desired ratio F.sub.kx /F.sub.ky can be determined, for example, according to the following equation: ##EQU5## wherein F.sub.a is the braking force; y.sub.k and x.sub.k are the relative vertical and horizontal position of the semitrailer coupling with respect to the center of gravity S of the semitrailer tractor; and wherein x.sub.1, x.sub.2 and y.sub.12 are the respective relative position of its axles to the center of gravity S. In this case, it should be observed that, in the case of a semitrailer tractor, in contrast to a normal truck, the values of x.sub.1, x.sub.2 and y.sub.12 are known because a semitrailer tractor (with the exception of negligible quantities, such as the tank content, etc.) always has approximately the same weight so that these values are vehicle specific and therefore need to be determined only once.
Since, as explained above, the value F.sub.a of the braking force has an uncertainty which cannot be avoided, it is, however, recommended that, the following equations be used for calculating the ratio F.sub.kx /F.sub.ky : ##EQU6## the constants N.sub.1, K.sub.1 and K.sub.2 being defined as follows: EQU N.sub.1 =m.sub.z (gx.sub.1 -ay.sub.k)+F.sub.2 (x.sub.2 -x.sub.1) EQU K.sub.1 =y.sub.y -y.sub.12 EQU K.sub.2 =x.sub.1 -x.sub.k
It is demonstrated that in this manner the ratio F.sub.kx /F.sub.ky can be determined without the use of the uncertain measurable value F.sub.a so that the result is correspondingly precise. Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.